The present invention relates to a furnace for firing wire-like products, and in particular copper wires enamelled with a cladding of insulating plastics resin.
As is known, furnaces currently utilised for firing the cladding of copper wires are of the continuous tunnel type and comprise at least one chamber of elongate form in which the wire is advanced longitudinally. In a first portion of this chamber evaporation of the solvents which impregnate the resins of the cladding take place; in a second portion of the chamber, maintained at a higher temperature than that of the first portion, polymerisation and cross linking of the resin itself take place.
Since the solvent vapours are pollutants the furnaces generally include an auxiliary unit which draws in a mixture of air and the said vapours at the end of the first portion of the chamber and directs it to a heating unit in which this mixture reaches the combustion temperature of the said vapours before being delivered to the chimney or at least part reintroduced, possibly mixed with air at ambient temperature drawn from the outside, into the principal chamber for controlling the temperature thereof.
In particular this heating unit generally includes an electrical resistance heater and a catalytic plate heater disposed in series with one another. The resistance heater heats the mixture of air and vapours up to a temperature sufficient to trigger the combustion of the vapours themselves, after which the catalytic plate encourages the complete oxidisation of these vapours into harmless combustion products (carbon dioxide and steam).
The fundamental disadvantage of heating units of this type is the limited durability of the catalytic plate, which must be periodically replaced; this involves stopping the furnace for several days. Moreover, the mixture cannot be heated by the electrical resistance to a temperature greater than a certain threshold, normally at around 600.degree. C., in certain cases 700.degree.-750.degree. C., above which the catalytic plate is damaged; this temperature limitation does not allow the use of certain enamels which could on the other hand be conveniently utilised, in that complete combustion of the solvent vapours would not be obtained. Finally, the catalytic plate does not allow the combustion temperature to be precisely regulated in dependence on the various types of enamel used.